The traditional approach to aerodynamic testing consists of installing a suitable model of a configuration under study in a wind tunnel and forcing air around it. In the presence of the flow, a variety of physical quantities such as forces, moments, and pressures acting on the model are measured. As well, properties of the flow around the model, such as local velocity, are observed by means of flow visualization and diagnostic techniques.
A frequently used alternate aerodynamic testing approach is based on the use of a water-tunnel. A water tunnel generally consists of an open channel through which water flows. The model is submerged in the water tunnel, resulting in a situation similar to a wind tunnel except that, in addition to the difference in test medium, the test medium has a free surface at its top surface.
A traditional tow tank can also be used in aerodynamic and hydrodynamic testing. Such tow tanks, known to someone skilled in the art, usually consist of a water channel containing a stationary liquid, in which a model is either submerged or floating on the surface, depending on the test being performed. A trolley system is set up over the channel and pulls the model through the channel, thereby imparting movement to the model. A less common towing tank is described in U.S. Pat. No. 4,587,841 to Giovachini et al. This towing tank can be used in hydrodynamic testing. The model floats or is submerged in water contained inside an elongated basin, and is supported by a mechanism within the tank that imparts the required motion. Giovachini et al. discloses a submerged frame, equipped with rollers and a motor, which is attached to the bottom of the tank. The model travels along the tracks at the desired velocity. Towing tanks, like water tunnels, also have a free top surface.
The free surface, present in both water tunnels and towing tanks, causes undesirable waves, turbulence, and other disturbances in the surface of the medium. These disturbances can cause changes in the pressure distribution within the tank, thereby altering the flow and affecting other measurements. External factors, such as vibration due to vehicular traffic, as well as the movement of the model itself, can also adversely affect the conditions in a tank or tunnel with a free surface. To minimize the aforementioned effects, conventional towing tanks filled with water are made sufficiently deep so as to allow positioning of the model far from the free surface.
In order to have a higher degree of control over the experimental conditions and avoid the problems associated with free surface disturbances, it is desirable to have a closed channel with no free surface. Such an apparatus would have the additional benefit of requiring less test fluid, an important consideration when dealing with costly liquids. The testing tank described in JP 11-344415 to Toyama et al. is capable of being converted from an open channel to a closed channel, by attaching a lid to the apparatus. However, once the channel is closed, there is no mechanism to impart movement to the submerged model, and free surfaces may still be present.
The disadvantages of conventional wind and water aerodynamic testing apparatus become even more apparent when it is desired to test at low Reynolds number flows. The Reynolds number is proportional to the ratio of the inertial forces and the viscous forces affecting the model. Testing at low Reynolds numbers is particularly important to support fundamental research, as well as the development of vehicles operating in such a flow regime, such as Micro Air Vehicles (MAV), high-altitude, long-endurance (HALE) vehicles, etc.
The experimental determination of airloads at low Reynolds number flows is extremely difficult with conventional apparatus, because the generated loads are so low as to be virtually immeasurable with an acceptable accuracy. Furthermore, the small size of the models required to carry out the tests, and correspondingly small associated flow features, seriously limits the accuracy and resolution with which the flow surrounding the test vehicle can be investigated. In addition, the rapid rate of growth of boundary layers at low Reynolds number conditions precludes the use of a test facility where the fluid moves with respect to the surrounding walls, as in the case of a wind or water tunnel, because the wall boundary layers prevent the development of a suitable velocity profile in the test section. It, therefore, becomes necessary to use an approach where the fluid is stationary with respect to the surrounding walls and the model is moved through it.
It is, therefore, desirable to provide a test apparatus or facility that overcomes the problems associated with the use of conventional apparatuses, particularly for testing at low Reynolds numbers.